Tanning, or sun-induced skin darkening due to increase production and dispersion of melanin within the epidermis, is widely regarded as a major endogenous protection against photocarcinogenesis. Although dosimetry, action spectrum, time course, and associated ultrastructural phenomena have been well studied, the mechanism by which UV irradiation stimulates pigment production in melanocytes is completely unknown. Using a recently developed culture system and in vitro irradiation protocol, we will test the hypothesis that UV-induced melanogenesis occurs via a retinoid- dependent rhodopsin-like phototransducer in the melanocyte. Newborn human foreskin melanocytes and S91 mouse melanoma cells (in preliminary experiments) will be repeatedly exposed to physiologically relevant doses of solar-simulated UV under retinoid deficient culture conditions and in the presence of known amounts of retinoid, retinoic acid, retinaldehyde, and retinoid antagonists; and monitored for UV-induced melanogenesis by measurement of tyrosinase activity and melanin content relative to each other and to non-irradiated controls. Indirect immunofluoresence of cultured cells and immunoblots of cell proteins will be used to detect binding of antisera raised against bovine ocular rhodopsin and transducin, a rhodopsin-associated g protein. Incorporation of radiolabeled retinoids into relevant cell proteins will also be determined. Further studies will examine the role of cyclic nucleotides, known to be involved in both ocular phototransduction and non-UV-induced melanogenesis; as well as the possible activation of bovine transducin by UV-irradiated membrane preparations and conversely activation of melanocyte- derived g protein in membrane preparation by purified irradiated ocular rhodopsin. Northern blot analysis will be used to detect mRNA capable of hybridizating with c-DNA probes for retinal visual pigments. The possible role of pre-vitamin D3 its photoisomers or metabolites as alternative melanocytic phototransducers will also be examined. These studies will substantially advance our understanding of human melanin production, particularly its stimulation by sun exposure.